1. Field of the Invention
This invention relates generally to a downhole measurement tool circuit and method to balance fault current in a protective inductor, and more particularly to a downhole measurement tool circuit and method to balance fault current in a protective inductor which keeps an alternating current balanced in a protective choke during a phase-to-ground fault condition in a power cable or a downhole motor of an electrical submersible pump.
2. Description of the Related Art
Various types of downhole equipment, such as pumps and similar devices, are used to move fluids from beneath the surface of the earth to the surface. Well known applications include oil and gas wells and water wells. A typical downhole arrangement would include a string composed of a series of tubes or tubing suspended from the surface. One type of well-known pump is a downhole electrical submersible pump. The electrical submersible pump either includes or is connected to a downhole motor which is sealed so that the whole assembly is submerged in the fluid to be pumped. The downhole motor is connected to a three-phase power source at the surface and operates beneath the level of fluid downhole in order to pump the fluid to the surface.
In the common design of many downhole measurement tools associated with an electrical submersible pump, they are connected to the Y-point of the downhole motor of the electrical submersible pump and to the ground of the downhole system, such as disclosed in U.S. Pat. No. 6,176,308, which is incorporated herein by reference. The three-phase power supply for the electrical submersible pump is isolated from the ground, and the downhole measurement tool utilizes this feature to communicate to an associated surface equipment of a downhole system by low frequency modulation of a current or voltage supplied by the associated surface equipment. The downhole measurement tool is coupled to the electrical submersible pump and used to monitor certain downhole parameters, such as pressure and temperature, of a subterranean bore-hole.
If a phase-to-ground fault occurs in the downhole motor or power cable of the downhole system, this will apply high voltage alternating current to the Y-point of the downhole motor of the electrical submersible pump, and consequently to the downhole measurement tool. To prevent damage to the downhole measurement tool during this fault condition, a protective choke is typically included in the circuitry of the downhole measurement tool, which provides a suitably high impedance to minimize the alternating current flowing from the Y-point to the ground of the downhole system through the circuitry of the downhole measurement tool.
Another typical requirement of many downhole measurement tools is to be able to use a negative polarity, direct current megger to check the electrical insulation quality of the power cable and/or downhole motor of the downhole system. For this purpose, the circuitry of the downhole measurement tool typically includes a diode, which only conducts during positive polarity voltage. However, since the diode only conducts in the positive polarity voltage, during a phase-to-ground fault of alternating current voltage, the current in the protective choke will reach a direct current level which saturates the choke, and lowers its inductance, accordingly reducing its protective function. In order to eliminate this saturation condition, a large capacitor is typically included in the circuitry of the downhole measurement tool in order to keep the choke current balanced during a phase-to-ground fault condition. The voltage and temperature ratings, plus the large physical sizes required by suitable chokes and capacitors cause them to be expensive and physically large, which may involve additional mechanical mounting considerations as described by U.S. Pat. No. 6,176,308.
It is therefore desirable to provide a downhole measurement tool circuit and method to balance fault current in a protective inductor that keeps a protective choke current balanced in the event of a phase-to-ground fault in a power cable or a downhole motor of an electrical submersible pump of a downhole system.
It is further desirable to provide a downhole measurement tool circuit and method to balance fault current in a protective inductor which eliminates the need for a large, expensive capacitor for protection against choke saturation, thereby reducing the size and cost of the downhole measurement tool.
It is still further desirable to provide a downhole measurement tool circuit and method to balance fault current in a protective inductor which causes a conducting of current during the negative polarity voltage portions of a phase-to-ground fault condition, but which do not cause a conduction of negative polarity voltage during use of a negative polarity megger.